1. Field of the Invention
The invention in general relates to the fabrication of integrated circuits, and more particularly to the fabrication of capacitors and transistors utilizing a metal oxide, such as barium strontium titanate, as the dielectric on silicon germanium substrates.
2. Statement of the Problem
A crucial problem of ULSI (ultra-large-scale integration) DRAMs (dynamic random access memories) is how to provide sufficient storage capacitance in the small memory cell area available. In conventional DRAMs, this problem is addressed by providing capacitors with highly complex structures to maximize the area of the capacitor. Such complex capacitors require complex fabrication processes, leading to decreased yields and increased cost. An alternative proposed solution is to use dielectric materials of high dielectric constant. Metal oxide materials, such as barium strontium titanate, commonly referred to as BST, are known to be useful in making integrated circuit thin film capacitors having high dielectric constants. See, for example, Kuniaki Koyama et al., “A Stacked Capacitor With (BaxSr1-x)TiO3 For 256M DRAM” in IEDM (International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. However, both these references use sputtering to deposit the BST, which is inherently hard to control. Such conventional processes, while useful in conventional silicon technology, when used in combination with capacitors that use metal oxides, such as BST, as the dielectric, result in capacitors that have relatively high leakage current, fatigue significantly, and generally have undesirable electrical properties. In particular, while the dielectric constant of bulk BST is of the order of 300 to 4000, the dielectric constant of thin films made according to the conventional processes is significantly lower. This is believed to be due to surface charges caused by defects and impurities in the films.
The process of spin coating has been used for making metal oxides such as barium titanate, strontium titanate, and barium strontium titanate. See G. M. Vest and S. Singaram, “Synthesis of “Metallo-organic Compounds For MOD Powders and Films”, Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42; Robert W. Vest and Jiejie Xu, “PbTiO3 Thin Films From Metalloorganic Precursors”, IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, November 1988, pp. 711-717; and “Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method”, Materials Research Society Bulletin, October 1989, pp. 48-53. In these papers, it was speculated that this process might have limited use for electronic devices; however, the grain size reported was 2000 Å minimum, which is as large or larger than the film thickness typically used for capacitor dielectrics in integrated circuits, and the quality of the film was inferior to that produced by other processes, such as sputtering, and thus the spin-on process as applied to these metal-oxides up to now has been limited to inks for screen printing and other macroscopic purposes. A paper given by some of the present inventors disclosed using a spin-on process to deposit BST, but does not disclose any further details of the process, and the spin-on process used yields inferior results to other fabrication processes. See “Deposition of Ba1-xSrxTiO3 and SrTiO3 Via Liquid Source CVD (LSCVD) For ULSI DRAMs”, given at the 1992 International Symposium on Integrated Ferroelectrics, Mar. 9-11, 1992.
Another problem associated with common integrated circuit substrates is that silicon from the substrate can migrate to the metal oxide layer of capacitors formed on the substrates. Silicon that has migrated into the metal oxide layer forms SiO2 within the metal oxide layer, thereby changing the capacitance properties of the thin film high-capacitance capacitors. It would be desirable to have a barrier diffusion layer that prevents this migration of silicon into the metal oxide layer of these integrated circuit capacitors.
Recently, it has been shown that ferroelectric transistors can make excellent memories. A requirement of such memories is that the dielectric separating the ferroelectric and the gate and the ferroelectric and the channel have a relatively low dielectric constant, but have very low leakage current and a capacitance that varies little out to frequencies up to about 10 gigahertz.
Further, it would be highly desirable to have an integrated circuit dielectric material and capacitor structure that was relatively simple, utilized conventional integrated circuit materials, and still performed well at high frequencies.
3. Solution to the Problem
The present invention solves the above problems by providing metal oxide thin films on a silicon germanium substrate in which the grain size is much smaller than the thickness of the film. The average grain size of the BST grains in the thin films according to the invention is about 40 nm. Typical film thicknesses of the dielectrics in integrated circuit capacitors are 100 nm to 200 nm. Thus, the grain size in the thin films according to the invention is 2.5 to 5 times smaller than the film thickness. This small grain size results in capacitors with much improved electrical properties.
The process according to the invention preferably includes deposition of a liquid precursor by a spin-on process. Preferably, the liquid precursor is an alkoxycarboxylate precursor as described in U.S. Pat. No. 5,514,822, which is hereby incorporated by reference. The liquid precursor is dried and annealed to form the BST. Preferably, the annealing is performed at a temperature between 600° C. and 850° C.
In an exemplary embodiment, capacitors made with a dielectric material comprising BST thin films having the formula Ba1-xSrxTiO3, where x is 0.3, were found to have a dielectric constant of nearly 500 that was nearly flat out to about 10 gigahertz and a leakage current of about 10−9 amps/cm2 when made by the process of the invention. The dielectric constant is about twice as large and the leakage current is about ten times as small as the best respective properties reported in the prior art for BST thin films. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.